Add GetSources to VideoRtpReceiver

BUG=webrtc:9770

Change-Id: I16143fce6eb727bbab0f6c621aa5b51bc6d28d6b
Reviewed-on: https://webrtc-review.googlesource.com/101600
Reviewed-by: Seth Hampson <shampson@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Reviewed-by: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Jonas Oreland <jonaso@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24858}

video/video_receive_stream.cc

@@ -459,5 +459,10 @@ bool VideoReceiveStream::Decode() {
   }
   return true;
 }
+
+std::vector<webrtc::RtpSource> VideoReceiveStream::GetSources() const {
+  return rtp_video_stream_receiver_.GetSources();
+}
+
 }  // namespace internal
 }  // namespace webrtc